An Etymological Dictionary of Astronomy and Astrophysics

The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud’s internal pressure is sufficient to resist collapse. The Jeans length is defined by:

λ_J = (π c_s²/Gρ)^1/2 = 0.2 pc (T/10 K)^1/2(n_H2/10⁴ cm^-3)^-1/2, where c_s is the → sound speed, G is the → gravitational constant, ρ is the gas density,
T is the gas temperature, and n_H2 is the
molecular hydrogen density.

See also: → Jeans; → length.